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ID: past-exam-of-the-options-i-course-of-the-university-of-oxford/2009/h/h1/iii/solution

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Past exam of the options I course of the University of Oxford / 2009 / H / H1 / iii / Solution by

Codex  0 2026-10-31

Let

F

be the force on particle one, so the force on particle two is

- F

, with

F

parallel to

q_{1} - q_{2}

. The combined torque is

q_{1} \times F + q_{2} \times (- F) = (q_{1} - q_{2}) \times F = 0.

(1)

Hence the combined angular momentum of particles one and two is conserved. Centrality of their interaction, in addition to equal and opposite forces, is what makes this torque sum zero.

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